Constant-current glow-discharge microphone



Dec. 27, 1927. 1,654,352

k I //z z Current in M/W/ki/npe r5. WITNESSES: INVENTOR Patented Dec. 27, 1927.

PHILLIPS THOMAS, OF EDGEWOOD, PENNSYLVANIA, ASSIGNOR TO WESTINGHOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION OF PENNSYLVANIA.

CONSTANT-CURRENT GLON-DISCHARGE MICROPHONE.

Application filed J'anuary 25, 1924. Serial No. 688,378.

My invention relates to telephone systems and apparatus and ithas particular relation to sound wave translating devices.

Broadly speaking, the object of my inven- 5 tion is toprovide an improved translating device for converting vibratory motionm a fluid medium into'vibratory electric currents.

More specifically, the object of my invention is to provide a telephonic transmitter device whereby sound waves are more easily translated into correspondingly modulated electrical energy.

Another object of my invention is to pro vide a telephonic translating device which shall be characterized by its simplicity of design and operation and which shall lend itself to inexpensive production.

Another object of my invention is to provide a telephonic translating device of the glow-discharge type wherein the intensity of response to pressure variations 1s substantially independent of the frequency of such variation.

A further object of my invention is to provide a glow-discharge telephone transmitter system which is adapted to provide a maximum sensitivity of response to sound waves.

My invention provides means whereby the current in the glow discharge may be held at a value at which there is secured maximum sensitivity coupled with maximum stability.

In my copending application, Serial No. 600,822, filed November 14, 1922, I describe a glow-discharge sound-sensitive device and the various phenomena involved in the sensitivity of a glow discharge to vibratory influences, such as sound waves.

In my copending application, Serial No. 688,397, filed January 25, 1924, and assigned to the Westinghouse Electric & Manufacturing Company, I describe means for increasing the sensitivity of a glow-discharge transmitter by the use of a chamber surrounding the positive electrode and the positive column portion of the glow discharge.

Further experiments have shown that the sensitivity of the glow discharge to sound vibrations is a function of the amount of current passing in the discharge. I find that there is a critical value of current passing, which is at about one-half milliampere, below which the glow it not maintained in a condition of steady. operation. I find that at currents just above this critical value a maximum sensitivity of response is obtained, and I find that, as the amount of current passing is increased to higher values, the sensitivity of the glow discharge to sound vibrations is markedly diminished.

It is therefore desirable that the amount of current flowing should be held as near as possible to the threshhold limit of critical current magnitude which will just maintain the discharge in steady operation. My invention provides means whereby this result is secured and, also, means for maintaining the quietness and stability of the discharge.

Other objects and structural details of my invention will be apparent from the following description when read in connection with the accompanying drawings, wherein:

Fig. 1 is a diagrammatic view of appa ratps and circuits embodying my invention, an

Fig. 2 is a curve showing the change of sensitivity of the glow discharge to sound vibrations of constant amplitude with change in current flowing in the discharge.

In Fig. 1, I have shown a glow-discharge transmitter 1, an amplifier 2, a constantcurrent device 3, a filter device t, a rectifier device-5, a current supply transformer 6 and a source of alternating current 7. The transformer 6 is constructed with a primary adapted to the alternating-current supply from which it is desired to operate the systern and a secondary giving a high voltage to feed the rectifier 5, which may be, as indicated, a full-wave thermionic discharge rectifier.

The filter 4 may comprise any desired combination of inductance and capacity which will produce a partial smoothing out of the fluctuations in the rectified currentsupply. It is not necessary that a complete filtering action takes place. Furthermore, the filter 4 m y. n Occasion, be omitted without serious detriment, but its presence is convenient.

The constant-current device 3 comprises a thermionic discharge device having an anode 11, a cathode 12 and a cathode heating and control means 13. The electrodes of the constant-current device 3 are insulated from ground, its cathode being connected to the negative terminal of the filter 4 and its anode being connected to the cathode of the glow-discharge transmitter 1.

The electrodes 11 and 12 are mounted within an evacuated container 14, being sealed through the glass walls at opposite ends of the device, thereby roviding for maximum current creepage distance for insulation between the anode'and the cathode. The container 14 is further surrounded by a conducting band 15 so placed as to surround the entire container between the points of entrance of the anode 11 and the cathode 12. The band 15 is then connected by a conductor 16 to the positive side of the circuit. The distance between the point of scaling in of the anode 11 and the band 15, and the scaling in of the cathode 12 and the band :15 is made sufficiently great to prevent sparking across and current leakage.

The band 15 further serves the purpose of maintaining a substantially constant electrostatic field within and surrounding the constant-current device. Its presence as a metallic conductor upon the exterior surface prevents current conduction by moisture films on the container surface between the anode and cathode, and thereby prevents sudden rushes of current which are audible in the connected transmitter as a sharp click. The band 15, being also connected to the positive side of the circuit, is strongly and positively charged with respect to the above anode and cathode of the constant current device. The presence of such a strong positively electrostatic field aids, increases and stabilizes the electron stream from cathode to anode.

The glow-discharge transmitter 1 and the amplifier 2 are constructed in accordance with the disclosure contained in my previously-mentioned applications a'nd connected,

as indicated in Fig. 1, to the other elements of my invention.

In Fig. 2, the abscissae represent current, milliamperes being plotted. The ordinates represent vibratory current output of the glow-dischar c transmitter in response to sound vibrations. They are plotted in empirical units. The curve represents change in response of the transmitter to a constant sound volume with change in current.- fiow in the discharge and is shown dotted for values below one-half millian'ipcres, since the discharge unstable with a current fiow below this value.

In the operation of: my invention, alternate ing currentis supplied to the primary of transformer 6 by tl source 7, stepped up to a suitably high vwhich may be in the neighborhood of 8000 to 4000 volts, and rectified by the thermionic. discharge rectifier 5 into a pulsating direct current.

Thg'pulsations in the rectified current are partially or wholly smoothed out by the filter organization 4. A current which may be slightly fluctuating or steady is thereby supplied to the constant-current device 3.

The temperature of the cathode 12 of the constant-current device 3 is raised by current from the source 13 under the control of the indicated rheostat. The tem'peratureof cathode 12 is raised until an electron-emitting temperature is reached, whereupon current may traverse the device 'by means of the electron stream produced. This current also traverses the series-connected glowdischarge transmitter 1, energy for the current flow through both devices being furnished by the high-voltage supply.

The high voltage from rectifier 5 will cause saturation current to traverse device 3. The value of this current is determined by the temperature of the cathode 12, inasmuch as the temperature determines the magnitude of the electron stream emitted. Adjustment of the cathode temperature thereby adjusts the value ofthe current.

This arrangement also serves to prevent fluctuations of current which may pass filter 4 from entering the glow-discharge transmitter 1. The current traversing the device 3 isv limited by the electron emission of the cathode 12. Sufficient voltage is applied, by the combination comprising the transformer 6, the rectifier 5 and the filter 4, to

current flow, since it is still above the value.

re uired to produce saturation.

y this means, the current through glowdischarge transmitter 1 is held constant at a magnitude just above the critical threshhold value required for maximum sensitivity, and fluctuations of current from the supply source are prevented from influencing the connected glow-discharge transmitter.

The fluctuations of voltage drop across the transmitter l produced by the incidence upon it of sound waves cause an alternating current to traverse condenser 17, which is connected between the glow-discharge transmitter 1 andthe amplifier 2, into amplifier 2, by which it is amplified. The output transformer 18 then delivers the amplified vibratory current to additional devices, as desired.

It is to be understood that the system shown and described is equally adapted to be employed with radio apparatus or with the usual telephone apparatus embodying con ductors.

While I have shown only one embodiment my invention in the accompanying drawings, it is capable of various changes and modifications Without departing from the spirit thereof, and it is desired, therefore, that only such limitations shall be placed thereon as are imposed by the prior art or set forth in the. appendcdclaiins.

I claim as my invention:

1. In combination, a glow-discharge transmitter, and a constant-current means C0111- prising a thermionic discharge device operated at its saturation current and connected in series with said transmitter, said thermionic discharge device comprising an evacuated container, an anode and a cathode therein, and a guard ring surrounding said container, said guard ring being connected to another point on said circuit.

2. In combination with a glow-discharge transmitter, a constant-current means comprising a thermionic discharge device operated at its saturation current, serially connected in circuit with said transmitter, said device comprising a container, an anode, a cathode and a constant-potential shield surrounding said container.

In testimony whereof, I have hereunto subscribed my name this 17th day of Jan uary 1924.

PHILLIPS THOMAS. 

